This invention relates to polymers with controlled narrow molecular weight distributions, and a polymerization process for producing such compositions. More specifically, the present invention relates to telechelic, branched and star polymers, and a process for preparing the polymers using an initiator/capping reagent to initiate the polymerization reaction and reversibly cap the propagating chain. This invention also relates to a method for preparing the initiator/capping reagent and then conducting the polymerization process either with or without isolating the reagent. The process is particularly useful in the production of thermoplastic polymer resins for use in a wide variety of thermoplastic applications.
The thermoplastic polymer resin compositions of the present invention may be formed into a variety of thermoplastic products, for example by known processes such as injection and blow molding processes. Examples of such thermoplastic products include resins for electrostatographic toner and developer compositions; narrow polydispersity polymers for applications including, but not limited to, adhesive formulations, surfactants and viscosity modifiers; and monomodal or multimodal resins for use in thermoplastic films and coating technologies.
One way to prepare polymers or copolymers having a narrow molecular weight distribution or polydispersity is by anionic processes. However, the use and availability of resins having narrow polydispersities in industrial applications is limited because anionic polymerization processes must be performed in the absence of atmospheric oxygen and moisture, and because they require hazardous initiator reagents that are difficult to handle. Consequently, such anionic polymerization processes are generally limited to batch reactors. In addition, the monomers and solvents that are used must be anhydrous and of high purity, rendering the anionic process more expensive than alternative processes that do not have these requirements. Thus, anionic polymerization processes are difficult and costly. It is therefore desirable to have a free radical polymerization process that would provide narrow molecular weight distribution resins that overcome the shortcomings and disadvantages of the aforementioned anionic polymerization processes.
Free radical polymerization processes are chemically less sensitive to impurities in the monomers or solvents typically used and are completely insensitive to water. Consequently there has been a long felt need for an economical free radical polymerization process that is suitable for preparing narrow polydispersity resins by suspension, solution (bulk or neat), emulsion and related processes.
Most copolymers prepared by free radical polymerization processes have broad molecular weight distributions or polydispersities, for example greater than about four or five. One reason is that free radical initiators have half lives that are relatively long, from several minutes to many hours, and polymeric chains are not all initiated at the same time. As a result, the free radical initiators provide growing chains of various lengths at any given time during the polymerization process. Another reason for the relatively high polydispersities is that the reaction rate in most free radical polymerizations is very high from the start of the reaction, thus quickly forming high molecular weight polymer chains. The propagating chains then tend to react with each other in processes known as coupling and disproportionation, both of which are chain terminating reactions. In doing so, chains are terminated at different times during the polymerization reaction process, which results in resin products having a high polydispersity. Furthermore, "dead" (non-reactive) polymer is formed very early in the reaction process, sometimes within milliseconds of initiation of the reaction, thus producing polymer in the early stages of the reaction having molecular weights that are different from the molecular weight of polymer formed at the end of the reaction. The different molecular weight polymers are generally caused by changes in process conditions during the course of the reaction, such as changes in the viscosity/monomer concentration and heat transfer properties of the reaction medium. The result is a further broadening of the polydispersity of resins prepared in the conventional way.
If a free radical polymerization process is to be enabled for producing narrow molecular weight distributions, then all polymer chains in the reaction must be initiated at about the same time and premature termination by coupling or disproportionation processes must be avoided.
In a hypothetical free radical polymerization of styrene, in which chains are continually initiated over the course of the polymerization, and where chain termination by coupling processes is also occurring, calculations have shown that the narrowest polydispersity that one can theoretically possibly obtain is 1.5. Such calculations are described in, for example, G. G. Odian, Principles of Polymerization, pages 280-281., 2nd Ed., John Wiley & Sons, 1981. In practice, polydispersities greater than 1.5 are actually obtained. Polydispersities of between 2.0 and 2.4 are typical for free radical homopolymerizations of styrene. In the case of copolymer systems, polydispersities of greater than 4 are generally obtained.
The use of stable free radicals as inhibitors of free radical polymerization is known and is described, for example, in G. Moad et al., Polymer Bulletin, vol. 6, p. 589 (1982). Studies have also reported on the use of stable free radicals as inhibitors of free radical polymerization performed at low temperatures and at low monomer to polymer conversation rates. See, for example, G. Moad et al., Macromol Sci.-Chem., A17(1), 51 (1982).
The stable free radicals are generally formed from precursor materials according to known reaction mechanisms. For example, the synthesis of nitroxides from amine precursors is described in E. G. Rozantsev and V. D. Sholle, Synthesis, 190-202 (1971) and E. J. Rauckman, G. M. Rosen and M. B. Abou-Donia, Synthetic Communications, 409-413 (1975). Other procedures for the synthesis of nitroxide include, for example, the oxidation of hydroxylamines, such as described in E. G. Rozantsev, Free Nitroxyl Radicals, 70-73 (Plenum Press, New York), and radical addition to Nitrones, for example as described in I. Iwamura and N. Inamoto, Bulletin of the Chemical Society of Japan, 40, 703 (1967). The disclosures of all of the preceding references are entirely incorporated herein by reference.
Roland P. T. Chung and David H. Solomon, "Recent Developments in Free-Radical Polymerization--A Mini Review," Progress in Organic Coating, vol. 21, pp. 227-254 (1992), presents an overview of the free radical polymerization process, with an emphasis on recent developments.
U.S. Pat. No. 5,322,912 to Georges et al. discloses a free radical polymerization process for the preparation of thermoplastic resins. The thermoplastic resins are disclosed as having a molecular weight of from 10,000 to 200,000 and a polydispersity of from 1.1 to 2.0. The process comprises heating a mixture of a free radical initiator, a stable free radical agent, and at least one polymerizable monomer compound to form a thermoplastic resin with a high monomer to polymer conversion ratio, and then cooling said mixture. The polymerization process is carried out at a temperature of from 60.degree. to 160.degree. C. and at a relatively low pressure of about 60 psi (about 4 bars). The process optionally comprises isolating the thermoplastic resin or resins and washing and drying the thermoplastic resin. The patent also discloses the preparation of mixtures and block copolymer thermoplastic resins using the free radical polymerization process. Resins produced by the disclosed process are described as having a narrow molecular weight distribution, and a modality that is controlled by the selection of the free radical initiator and stable free radical agent. As the stable free radical agent, the patent discloses the use of nitroxide free radicals such as PROXYL, TEMPO, and derivatives thereof.
U.S. Pat. No. 4,581,429 to Solomon et al. also discloses the incorporation of a stable free radical agent into a free radical polymerization process. The patent discloses that a nitroxide radical may be added to the polymerization process to stabilize the growth of polymer chains. The molecular weights of the polymer products obtained are generally from about 2,500 to 7,000 and have polydispersities generally of from about 1.4 to 1.8. The reactions typically have low monomer to polymer conversion rates and use relatively low reaction temperatures, of less than about 100.degree. C., and use multiple stages.
U.S. Pat. No. 4,581,429 discloses a free radical polymerization process that controls the growth of polymer chains to produce short chain or oligomeric homopolymers and copolymers, including block and graft copolymers. The process employs an alkoxyamine initiator having the formula, in part, .dbd.N--O--X, where X is a free radical species capable of polymerizing unsaturated monomers. The reference discloses that the alkoxyamine free radical initiator may be formed in situ prior to its use in a free radical polymerization process by heating a nitroxide radical in the presence of a stoichiometric amount of carbon centered free radical (X). For example, the reference discloses the formation of the alkoxyamine free radical initiator 1-(1-cyano-1-methylethoxy)2,2,5,5-tetramethylpyrrolidine from a degassed solution in benzene of azobisisobutyronitrile and 2,2,5,5-tetramethylpyrrolidin-1-yloxy (the stable free radical agent PROXYL, which is equivalent to the structure 2,2,5,5-tetramethyl-1-pyrrolidinyloxy specified for PROXYL below). The reference also discloses the formation of the alkoxyamine free radical initiator 1-(1-cyano-4-hydroxy-1-methylbutoxy)2,2,6,6-tetramethylpiperidine from a degassed solution in ethyl acetone of 4,4'-azobis(4-cyano-n-pentanol) and 2,2,6,6-tetramethylpiperidin-1-yloxy (the stable free radical agent TEMPO, which is equivalent to the structure 2,2,6,6-tetramethyl-1-piperidinyloxy specified for TEMPO below).
U.S. Pat. No. 4,777,230 to Kamath discloses a free radical polymerization process for producing polymers, wherein monomers are blended with a solvent, polymerization initiators (such as peroxide initiators) and an optional chain transfer agent. The polymerization process is conducted at a temperature of from about 90.degree. C. to about 200.degree. C. The resultant polymers have a molecular weight distribution of from about 1.5 to about 2.5, and an average molecular weight of less than about 4,000.
In free radical polymerization reaction processes of the prior art, various significant problems exist. For example, difficulties exist in predicting or controlling the polydispersity and modality of the polymers produced. These free radical polymerization processes may produce polymers with a high weight average molecular weight (M.sub.w) and a low number average molecular weight (M.sub.n), resulting in a broad polydispersity. Further, bulk or neat free radical polymerization processes of the prior art are prone to generating excessive quantities of heat since the polymerization reaction is exothermic and the dissipation of heat becomes more difficult as the viscosity of the reaction mixture increases. This is referred to as the Trommsdorff effect, as discussed and illustrated in G. Odian, Principles of Polymerization, 2nd Ed., Wiley-lnterscience, New York, page 272 (1981), the disclosure of which is entirely incorporated herein by reference. Moreover, the exothermic nature of free radical polymerization processes is often a limitation that severely restricts the concentration of reactants or the reactor size upon scale-up.
It has been demonstrated that stable free radical polymerization processes can provide precise control over the molecular weight distribution of polymer chains. For example, U.S. Pat. No. 5,322,912, described above, describes polymerization processes that use stable free radicals to provide thermoplastic resins having a narrow molecular weight distribution. Although it is not desired to be limited by theory, it is believed that when polymerization reaction processes are performed at temperatures above about 100.degree. C., all of the polymer chains are initiated at about the same time. Therefore, control of the reaction enables the formation of polymer chains having a precise molecular weight and a narrow molecular weight distribution. Incorporation of stable free radical agents in the polymerization process prevents the initiation of new polymer chains after an initial reaction during which all of the polymer chains are initiated at about the same time.